1. Field of the Invention
The present invention relates to an optical waveguide device having an optical waveguide and electrodes for controlling a guided lightwave which are formed on a surface of a substrate which has an electrooptical effect, and more particularly to an improved optical waveguide device which can be operated stably against an ambient temperature change and the properties of the optical modulator are not affected by unevenness in electric conductivity of antistatic films for preventing unstable operation by pyroelectrically induced charge or the like.
The present invention also relates to an optical waveguide device which is capable of a high speed operation and utilizes a substrate with an electrooptical effect, more specifically, to a constitution for optical intensity modulators, optical phase modulators, polarization scramblers or the like which improves properties of long distance, optical fiber transmission.
2. Description of the Related Art
In recent fiber optics communication systems, a broad bandwidth, high capacity transmission of several GHz to several ten GHz at long distance of several thousand kilometers has been studied and such transmissions have been partially turning into practical use.
Under such circumstances, optical waveguide devices are studied as the key device. Since a lightwave is confined in a waveguide in an optical waveguide device, a high speed operation is possible and miniaturization, integration, complexation and the like can be easily attained. Furthermore, it has also been investigated to apply an optical waveguide device to optical measuring instruments, optical information processing or the like.
Among the optical waveguide devices, those having an optical waveguide formed by effecting a thermal diffusion of the Titanium (Ti) into a ferroelectric substrate such as LiNbO.sub.3 (hereinafter simply referred to as LN) which has a great electrooptical effect are known and various investigations have been made on them. For an optical modulator utilizing such LN waveguide, it is relatively easy to suppress optical losses. Moreover, it has been proved that the optical modulator would have a modulation bandwidth of 10 GHz or more by constituting a hot electrode as a traveling-wave type electrode and thus, it is considered as a key device for realizing an super-high speed optical communication system.
FIG. 4 is a perspective view showing a waveguide-type optical phase modulator as an example of Ti-diffused, LN waveguide-type optical modulators.
In this optical modulator, Ti is diffused into a surface of Z-cut LN substrate and a traveling-wave type electrodes are formed thereon. Although omitted from the illustration in FIG. 4, a buffer layer which is, for example, composed of SiO.sub.2 is formed between a waveguide and the electrodes in order to prevent a guided lightwave from being absorbed in the metal electrodes.
To avoid operational fluctuations due to polarization change, an incident lightwave is polarized only one direction using a laminated micropolarizer lamipole (11), since the LN substrate is a large anisotropic crystal. The guided lightwave interacts with an electric field occurring due to microwave (electric signals) applied by a signal source (7), thereby changing the phase of guided lightwave.
For the purpose of further broadening the bandwidth, the electrodes are generally formed as a thick film in those modulators having a traveling-wave type electrodes. In other words, broadening of the bandwidth is attained by matching the propagation velocity of the guided lightwave with the velocity of the microwave traveling across the electrodes, namely, by velocity matching between the guided lightwave and the microwave. In general, such thick film electrodes are formed by an electroplating process.
Other than waveguide-type optical phase modulators, one example of which has been explained so far, mention may be made to branched interferometric-type optical intensity modulators in which two Y-branch waveguides are combined, as well as to polarization scramblers wherein a polarizer is arranged on the incident side of the optical phase modulator at an angle of 45 degrees with respect to the crystallographic axis.
Since a ferroelectric material is used as the substrate in these devices, they are likely to be pyroelectrically charged due to ambient temperature change, and what is worse, operation of the device becomes unstable because of such pyroelectrically induced charge.
The mechanism of such phenomenon will be explained below with reference to FIG. 5a and FIG. 5b.
In FIG. 5a, there is shown a structure in which no antistatic film is formed. In this case, an electric field caused by induced charges on the surface of a substrate (6) changes the refractive index of a waveguide (3) and thus, properties of the modulator become unstable with respect to ambient temperature change.
On the other hand, when an antistatic film (5) is formed as illustrated in FIG. 5(b), the electric field caused by the induced charges scarcely crosses the waveguide (3) and thus, the device can be quite stable against ambient temperature change.
In Japanese Patent Publication No. 5-78016, for example, there have been proposed a constitution as shown in FIG. 3(a), wherein a waveguide (3) and a buffer layer (4) are formed on a surface of a LN substrate (6), and after forming coplanar electrodes (1) and (2) in the form of thick film, an antistatic film (5) is formed all over the surface, and a constitution as illustrated in FIG. 3(b) in which an antistatic film (5) is formed all over the surface of a buffer layer (4) before the formation of coplanar electrodes (1) and (2).
The above-mentioned two constitutions have such a common feature that the antistatic film (5) is always in contact with both of the hot electrode (1) and the ground electrode (2).
This feature is aimed to uniformly distribute the charges induced in the surface of the substrate between the electrodes, and is considered to be necessary for preventing unstableness in operation of the device.
However, the pyroelectric charge is not the only cause for unstableness in operation of the device. The electrode films which are formed thickly for broadening the bandwidth of the device have a quite large stress. Since this stress fluctuates in accordance with temperature change, an operating point acting on the waveguide may be shifted via the photoelastic effect of the LN substrate. In addition, other stresses such as thermal stress of the buffer layer formed on the surface of the substrate also act on the waveguide through the photoelastic effect of the substrate, thereby shifting the operating point. Accordingly, even a constitution in which an antistatic film is formed all over the surface of the substrate involves such a problem that the device cannot be operated stably against the ambient temperature change.
Material such as ITO and Si are generally used for the antistatic film (5), but the electric conductivity of these materials is greatly affected by a very little difference in contained impurities or manufacturing conditions (it easily changes by several hundred times to several thousand times). Since such film is in contact with both of the hot electrode (1) and the ground electrode (2), the resistance between the electrodes of the modulator may be greatly changed, thereby causing fluctuations in electrical properties.
Moreover, it is known that when an electric field is introduced into a semiconductive film such as the antistatic film, the electric field strength is weakened by the conductivity of the film. Accordingly, there is such a problem that when an antistatic film is widely formed between the electrodes (1), (2) and the buffer layer (4) as illustrated in FIG. 3b, the electric field occurring by applied microwave to the hot electrode (1) is decreased due to the conductivity of the antistatic film.